Data rich imaging approaches assessing early fatigue crack growth mechanisms in Ni base superalloys

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Nickel base superalloys are used in turbine disc applications because of their excellent high temperature strength, oxidation resistance and damage tolerance. In service they experience extremes of temperature and cyclic loading conditions at relatively high stresses and so much of their fatigue life is controlled by the early stages of crack initiation and growth. The effect of varying' size on crack initiation and growth in nickel based superalloys is often linked to variation in slip character. The accumulation of damage is intrinsically affected by the degree of slip heterogeneity and this can affect both initiation and growth processes. Thus the combined effects of temperature, environment and slip character linked to' size control overall fatigue life through varying effects on initiation processes and crack growth mechanisms. Most in-service turbine disc alloys have a bi-modal or tri-modal' size distribution, here primary ' size (or presence) is linked to the duration and temperature of the solution heat treatment (e.g. super or sub-solvus) whilst secondary' and tertiary ' size are linked to the ageing temperature and the cooling rates. The effect and relative balance of these different ' size populations on slip character is a matter of continuing debate. In this work a non-commercial heat treatment has been used on RR1000 alloy to produce two different unimodal ' size distributions . This comprised a super-solvus heat treatment followed by a single age and slow cool to produce fine and coarse' sizes, respectively. Short fatigue crack tests have been conducted on polished plain bend bar of fine and coarse' materials by three point bending method under 90% and 110% of yield stress, load ratio of 0.1 and a sine waveform of 20Hz frequency at room temperature. Uninterrupted and interrupted tests with replication ethod to investigate crack evolution were carried out and compared with the previous work done by the University of Southampton. Plain bend bar tests showed that the RR1000 with fine variant had a slightly longer fatigue life than RR1000 with coarse ', linked to slower Stage I crack propagation linked to more planar slip at room temperature. Cracks mostly initiated at pores or slip bands at room temperature and fatigue crack growth rate of fine ' is slightly higher that of coarse'. 3D evaluation has been provided by X-ray CT of extracted match-sticks at University of Southampton and by inspecting crack initiation and growth in micro-tensile samples. Internal microstructure analysis of RR1000 with coarse' including porosity distribution and crack behavior has been investigated from a series of 2D slices obtained by scanning. This provided a deeper understanding of the mechanisms of fatigue crack nitiation and propagation behaviour in this model turbine disc alloy under a range of service relevant applications by the combination of 3D X-ray computed tomography and traditional 2D observations.